R. A. Fisher (1890-1962)

One of the most brilliant biologists ever. British geneticist, made major contributions to population genetic theory, wrote landmark Modern Synthesis book, invented many important statistics that are widely used not just in genetics, but alpha taxonomy, ecology, and non-biological fields. Fisher's models emphasized mass selection in panmictic populations and tended to assume that phenotypes results from many genes of small, additive effects.
This put him in direct conflict with the American neo-Darwinian theorist S. Wright, leading to a famous public debate that lasted three decades.